Where the function of an actuator is critical to safety, it is well known to incorporate redundancy into the system in which the actuator is used so as to accommodate failures within the actuator or associated control system without preventing operation of the system.
One form of valve suitable for use in controlling the operation of such an actuator takes the form of a linear control valve. In order to permit the provision of the required degree of redundancy in the event of the linear control valve becoming jammed, a separate by-pass valve arrangement is provided to permit the valve outlets to be connected to one another and so permit the associated actuator to be moved by an external device or under the control of another valve to a desired position. An alternative valve suitable for use in this type of application is a rotary valve comprising a pair of spools arranged concentrically within a sleeve. In normal use, angular movement of one of the spools relative to the other spool is used to control the delivery of fluid to the actuator. In the event that the spools become jammed so that such relative movement is no longer possible, adjustment of the angular position of the second spool relative to the sleeve can be used to achieve the desired level of control to permit continued operation.
A rotary control valve which is arranged to occupy a by-pass mode in the event of a jam is described in EP 2863069 (A1) and an actuator control arrangement using the rotary control valve is described in EP 2574797 (A2). A rotary control valve may also be provided with a test piston which is arranged to place the rotary control valve into a by-pass mode to ensure by-pass mode is available. The test piston is accessible only during maintenance.